This section is intended to provide a background or context to the invention that is, inter alia, recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
A fuel cell is an effective device for converting chemical energy to electrical energy through electro-catalytic reactions. The polymer electrolyte fuel cell (PEFC) operates at a relatively low temperature with the gas phase hydrogen as fuel and oxygen (air) as oxidant. At the core of a PEFC is the membrane electrode assembly (MEA) which includes an anode, a cathode, and a polymer electrolyte layer in between. Because of its relatively high conversion efficiency, low noise and low emissions, the PEFC is deemed to have substantial potential for use in a variety of applications, including automobiles and distributed power generation.
One type of PEMFC uses electrolyte that conducts alkaline ion such as hydroxyl ion, OH−. Such type of fuel cell is also known as alkaline fuel cell (AFC). AFC operates at a relatively low temperature with hydrogen or hydrocarbon as fuel and oxygen (air) as oxidant. Due to its high conversion efficiency, low noise and low emissions, AFC is deemed with high potential in a variety of power generation. Effective catalysts will be needed to promote the electrocatalytic reactions at both anode and cathode of AFC. Such catalytically promoted reactions will improve the overall fuel cell efficiency by reducing the overpotentials caused by the potential barriers from the electrochemical reactions.
At present, generally the most effective catalyst for electrocatalytic reactions utilizes a platinum (Pt) electrode catalyst supported on an amorphous carbon substrate. A typical Pt loading on the MEA surface ranges from about 0.2 mg/cm2 to about 0.4 mg/cm2. Because platinum is a precious metal with limited supply, its use as a catalyst adds a significant cost to a PEMFC system. Other platinum group metals (PGMs), such as Pd, Rh, Ru, etc., are being evaluated as a possible replacement for Pt. However, PGMs also generally suffer from high cost and limited reserves. As such, the use of PGMs in electrochemical devices such as a fuel cell typically adds significant cost to the system and represents a major barrier to commercialization. There is a strong need to find low cost materials as non-PGM catalyst to replace the usage of PGM materials therefore the overall cost of the fuel cell system.
Various attempts have been made to replace PGMs in fuel cells. These attempts have been mainly focused on developing replacement materials utilizing transition metal compounds. For example, it is known that molecules containing a macrocyclic structure with an iron or cobalt ion coordinated by nitrogen from the four surrounding pyrrolic rings have catalytic activity toward capture and reduction of molecular oxygen. Additionally, oxygen reduction reaction (ORR) catalytic activity can be improved for systems containing coordinated FeN4 and CoN4 macrocycles through heat treatment. Examples of a macro-molecular system containing FeN4 and CoN4 moieties include corresponding transitional metal phthalocyanine and porphyrin.
Methods of preparing non-PGM catalyst by incorporating a transition metal into heteroatomic polymers in a polymer/carbon composite are also known. Additionally, good ORR activity can be achieved by mixing amorphous carbon based catalyst with FeN4 group and carbonaceous material or synthetic carbon support, followed by high temperature treatment in a gas mixture of ammonia, hydrogen and argon. An iron salt adsorbed on carbon in the presence of a nitrogen precursor can also produce a catalyst with good ORR activity. However, such catalyst material will generally decompose under acidic conditions to release iron, and thus is unstable for the electro-catalytic reaction within a fuel cell cathode. Additionally, because carbon does not carry the electrocatalytic activity by itself, using a carbon support dilutes the catalytic active site and results in inhomogeneous active site distribution and low activity in the final catalyst materials thus prepared.
In light of these considerations, there is a need to develop a low cost alternative to PGMs as the electrode catalyst for fuel cells and similar electrocatalytic applications.